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On being cooled to temperatures well below the onset temperature for wax crystallization, mineral oils undergo a transition from viscous to elastic-solid (G‘ ≫ G‘ ‘) behavior, referred to as gelation. Gelation is caused by the formation of a wax crystal network and is distinct from a glass transition. We investigate the yield behavior of gelled mineral oils, linear elastic and fracture yield limits, as a function of the amount of crystallized wax and the particle size distribution of the wax by studying yield behavior as a function of temperature. By cooling the mineral oils at different constant cooling rates, we altered the surface-area-average crystal length (l s) in the wax crystal network. The observed yield behavior is compared to that reported for gels formed from flocculated or aggregated colloidal particles; gelled mineral oils exhibit a weak-link type of scaling in contrast to the strong-link scaling exhibited by flocculated colloidal gels.

Extension and surface coverage of adsorbed poly(2-vinylpyridine)/polystyrene (PVP/PS) diblock copolymers were studied by measuring their effect on convective transport in the pores of wellcharacterized mica membranes. When PVP/PS is adsorbed to a mica membrane from toluene, the PVP block anchors the diblock to the surface and the PS block imitates a terminally attached chain extending from the pore wall. The hydrodynamic thickness of the polymer layer in toluene was measured, and using a Debye-Brinkman model for the hydrodynamic interaction of the polymer segments with the fluid, it was concluded that the hydrodynamic thickness is a good measure of the polymer layer extension. This conclusion was further supported by agreement between the hydrodynamic thickness in toluene and the chain extension estimated from published force-distance measurements on the same diblock samples and by a constant hydrodynamic thickness over a range of shear rates (103-104 s_l). The polymer layer was collapsed by replacing toluene with the nonsolvent heptane, and the hydrodynamic thickness measurements in heptane were used to estimate the surface coverage. Reversibility of the extension of the polymer layer to changes in solvent quality, from toluene to heptane to toluene, was observed.

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